1. Field of the Invention
This invention relates to laser surgery and to cutting of dental and other hard tissue and non-cellular material.
2. Description of the Related Art
Laser use in dental enamel surgery was reported as early as 1964 using a ruby laser. Although such reports indicated that lasers could be used on dental hard tissue, lasers have not generally been used clinically until recently for surgical processes, including drilling teeth, because of the large amount of damage to nearby tissue that is often associated with such drilling. Pulsed eximer lasers as well as lasers producing infrared radiation have, however, been used recently for soft tissue and bone ablation due to the fact that these types of lasers have been found to do less damage than previous lasers.
Myers et al. patented a method for removing decay from teeth using a yttrium-aluminum-garnet (YAG) laser for a picosecond to several milliseconds (U.S. Pat. No. 4,818,230). The laser was used to eradicate tooth decay located in the dentin, "without significantly heating the tooth and thus without damage to the nerve". The disclosure of this patent and all other patents and publications referred to herein is incorporated herein by reference.
A YAG laser has also been used to remove incipient carious lesions and/or stain from teeth (U.S. Pat. No. 4,521,194). This use of a YAG laser was found to slightly fuse the crystals which form the tooth enamel and make the tooth enamel more impervious to decay.
Blum et al. (U.S. Pat. No. 4,784,135) discloses use of an ArF excimer laser as an ultraviolet light source (wavelengths less than 200 nm) to ablatively photodecompose decayed teeth and remove the surrounding enamel.
Erbium is a metallic element of the rare-earth group that occurs with yttrium and is also used as a source of laser irradiation. An Er:YAG laser is a solid-state, pulsed laser which has a maximum emission in the mid-infrared region at 2.94 um. Water absorbs strongly in this region with the water absorption coefficient for radiation produced by an Er:YAG laser being ten times that of radiation produced by a CO.sub.2 laser. Laser surgery performed with an Er:YAG laser apparently results in water in the target tissue absorbing radiant energy and heating to boiling to produce water vapor. The water vapor builds up in pressure at the surgical site until a microexplosion occurs and a small portion of tissue is ablated. A number of publications have discussed the great potential for Er:YAG lasers for tissue, bone and cartilage ablation (e.g., Laryngoscope 100:14, 1990; Lasers in Surgery and Medicine 8:494, 1988; 9:327, 1989; and 9:362, 1990). Radiation from a pulsed Er:YAG laser can be transmitted through optical fibers and its pulse nature allows cooling between pulses
Researchers in Germany have found that pulsed 2.94 um Er:YAG laser radiation in vitro is effective in removal of both dentin and enamel (Hibst and Keller, Lasers in Surgery and Medicine 9:338, 1989). These researchers found that when the duration of the total erbium laser pulse was about 250 microseconds with a pulse train of single spikes of about 1 microsecond each, roughly cone-shaped holes were produced. They also found that with a radiant exposure of 30 J cm.sup.-2, the depth hole in dentin and enamel was proportional to the number of pulses, except at higher numbers of pulses for enamel.
In a companion study, the same researchers used light and scanning electron microscopy to view tooth dentin and enamel exposed to Er:YAG laser radiation (Lasers in Surgery and Medicine 9:345, 1989). Using the same laser treatments as in the companion paper, they found that very few charred or fused zones or cracks were found with the Er:YAG treatment, as compared to CO.sub.2 laser dental surgery. There was also little heating of the tissue surrounding the crater.
Water has been used in conventional dental surgery and in laser dental surgery as a coolant for the tooth after a surgical pulse. For example, the patent of Vassiliadis et al. (U.S. Pat. No. 4,940,411) discloses a dental laser method using a Neodymium:YAG laser. In this invention, water is sprayed on the tooth after a pulse, followed by drying of the tooth prior to a subsequent activation of the pulsed laser. This patent and the work of others stress the importance of keeping the tooth dry during delivery of the laser pulse, especially for any lasers, such as an Er:YAG laser, productive of radiation that is absorbed by water to minimize heating of and damage to the surface of the tooth.
In contrast to the recited prior practices, the present invention is based on the surprising discovery that controlled addition of water, rather than drying of the surface, prior to and/or during laser surgery, so that no more than a thin film of water is present during surgery, results in a significant increase in laser efficiency and less residual damage than with the prior laser surgery methods for hard materials. In addition, the method of the invention based on the aforesaid discovery results in less heating of the tissue surrounding the surgical site than with prior methods of laser irradiation.
It is therefore an object of this invention to provide a method of laser surgery that results in increased surgical efficiency and decreased damage. It is a further object of this invention to provide a method of laser surgery that may be used for hard tissues such as dentin and enamel as well as other hard organic and inorganic substances
Other objects and advantages will be more fully apparent from the following disclosure and appended claims.